This invention relates to air foils, and more particularly to an air foil having a leading edge construction for reducing drag and improving heat sink properties of the air foil.
Aircraft operating at hypersonic speeds impose severe thermal and structural stresses on the wings and fuselage of the aircraft. The propulsive force required for the vehicle to maintain a constant velocity must equal the vehicle drag, and thus drag reduction is a very important consideration in aircraft design. It is well known that drag can be reduced by reducing the radius of the leading edge of the wings of the aircraft. However, it is equally well known that a reduction in the radius of the leading edge of a wing increases the amount of heat generated per unit area on the leading edge. For any given operating condition, as the radius of the leading edge decreases, the recovery temperature of the leading edge increases. Thus, the reduction of the radius of the leading edge has been limited by the thermal properties of the material from which the leading edge is constructed. The minimum size of the radius of the leading edge has therefore been limited by the maximum allowable operating temperatures that the materials from which the leading edge of the wing is constructed can withstand.
High lift/drag type vehicles require very sharp leading edges to reduce drag. From preliminary analysis and testing of a Mach 8 type vehicle, it has been determined that only the tip of the leading edge of an air foil suffers from overheating, and that a metallic leading edge resulted in a one thousand degree reduction in temperature. However, fabrication of the entire air foil leading edge from a metal that could withstand the temperature experienced during hypersonic flight speeds would result in an overall weight for the leading edge that is unacceptable for a hypersonic aircraft.
Composite air foil structures that could endure the heat experienced by a leading edge of an air foil during hypersonic flight have been fabricated by the assignee of the present application from a composite material comprising carbon/carbon, and also from other ceramic matrix composites (CMC) comprising carbon silicon carbide (C/SiC), but each was expensive and difficult to fabricate.
Prior approaches to managing the heat experienced by components of an aircraft during very high speed flight have met with limited success. In one approach disclosed in U.S. Pat. No. 3,776,139 a pyrolytic carbon nose for hypersonic vehicles was depicted. The graphite was arranged in slices and oriented so that the direction of least thermal conductivity was along the main axis of the nose, whereas the direction of greatest thermal conductivity was at right angles to the surface of each slice.
In a different approach shown in U.S. Pat. No. 4,667,906, a replaceable tip for an aircraft leading edge was depicted which had a metallic abrasion shield glove removably mounted to the leading edge of the aircraft. Fasteners were used for securing the abrasion shield glove to the leading edge of the aircraft.
In yet another approach disclosed in U.S. Pat. No. 4,966,229, a leading edge construction was depicted with a heat pipe arrangement. The heat pipe included an acute angle leading wedge shaped form with a radiused leading edge. The design used the aircraft engine fuel supply to cool the trailing wedge shape form.
In still another approach disclosed in U.S. Pat. No. 4,991,797, the invention provided a system for selective reduction of an infrared signature of a vehicle subjected to aerodynamic heating. Liquid coolant under pressure vaporized in porous sections of the skin of the vehicle to transpiration-cool the skin. Adjacent downstream solid skin sections were film-cooled by the vapor introduced in the boundary layer.
In U.S. Pat. No. 5,299,762, a leading edge construction included a relatively thin solid plate extending forwardly from the air foil. The plate had an exposed top surface and an exposed bottom surface, and a relatively sharply radiused forward edge forming the leading edge of the air foil. The construction further included slots adjacent to the top and bottom surfaces of the plate for injecting coolant over the top and bottom surfaces of the plate, towards the leading edge, to provide active cooling of the plate.
In U.S. Pat. No. 5,351,917, a transpiration cooling system for avoiding overheating of an air foil was depicted. The air foil was provided with a plurality of apertures and a source of pressurized fluid for providing a slower fluid through the apertures to establish an aerodynamic radius. The aerodynamic radius of curvature of the leading edge was greater than the mechanical radius of curvature of the leading edge such that peak heat flux was independent of the mechanical radius of curvature.
In another application, U.S. Pat. No. 5,772,154, a heat shield was disclosed for thermally insulating the leading edge of a wing of a spacecraft during ascent and re-entry, which included a plurality of rigid tiles. Each tile was formed with a pie-shaped element which interlocked with a complimentarily-formed element of another tile.
In spite of the teachings of the above-mentioned patents, there is still a significant need for a leading edge component which is able to withstand the high temperatures that result from travelling at high Mach speeds without significantly increasing the weight of air foil design.
It is therefore a principal object of the present invention to provide an air foil having a leading edge construction for reducing drag and improving heat sink properties of the air foil. The leading edge design would have the low weight benefit of a composite tile structure with the ability to withstand high temperatures experienced during high speed flight.
It is still another object of the present invention to provide a leading edge construction for an air foil that includes a composite material such as high density AETB formed to a predetermined aerodynamic shape, and a metallic component with high thermal conductivity secured to a forwardmost portion of the composite material to thereby form a xe2x80x9chybridxe2x80x9d leading edge component. This leading edge component would have the low weight benefit of composite tile construction with the ability to withstand the extremely high temperatures experienced during high speed flight.
It is still another object of the present invention to address the concerns regarding high temperature tolerance and low weight with a leading edge construction that is easily fabricated in a cost effective manufacturing process.
The present invention relates to an air foil having a leading edge component for reducing drag and improving the heat sink properties of the air foil. The leading edge component comprises a composite material formed in a predetermined aerodynamic shape and a metallic component with high thermal conductivity which is secured to a forwardmost portion of the composite material. This construction provides a leading edge component which is able to withstand the high temperatures that result from travelling at very high speeds without adding a tangible amount of weight to the overall air foil.
In a preferred embodiment of the invention, the apparatus of the present invention includes a composite tile component, preferably Reaction Cured Glass (RCG)/Toughened Unipiece Fibrous Insulation (TUFI) high density AETB tile or a suitable equivalent. A metal lip portion, preferably formed from Inconel or other suitable alloy, is secured to a forwardmost portion of the leading edge component. The metal lip portion is secured to the composite tile component by an adhesive, while the blade of the metallic insert is allowed to expand thermally in a slot machined into the AETB-tile.
In an alternative preferred embodiment the apparatus of the present invention comprises a composite tile component having at least one bore. A metallic lip portion has an enlarged portion which engages with the bore to enable the lip portion to be secured to the tile component without adhesives.